Diabetes mellitus is an increasingly common disease in the United States and one of its major complications, macular edema, is the most common cause of vision loss in the working-age population. A large national study (ETDRS) has indicated that treatment should be initiated based on the presence of retinal thickening. The study has also indicated that focal laser treatment reduces the risk of further loss in vision compared to lack of treatment. Unfortunately, the reduction is limited to 50 percent and 10 percent of treated patients who had moderately reduced vision improved after the treatment. At the present, there is no explanation to account for the fact that some patients with macular edema retain vision while other deteriorate or for the observation that some patients benefit from treatment while others do not. The lack of a sensitive and quantitative method to detect and monitor macular edema has prevented correlation of the degree and rate of progression of retinal thickening with the degradation in vision or the outcome of focal laser treatment. The applicant has developed an instrument, the Retinal Thickness Analyzer (RTA), to measure the retinal thickness, and have applied it to perform studies of macular edema. The results have indicated that edema is common even at early stages of diabetic retinopathy and that macular edema can cause thickening of the retina up to three times the normal thickness. In addition, when the thickening was pronounced, laser treatment was found to be less effective in reversing the edema. The main goal of the proposed continuation of the research is to perform two longitudinal studies to investigate the relationship between retinal thickening and vision outcome in diabetes. The first will consist of following 150 diabetic patients to establish the time course of retinal thickening and elucidate its temporal relationship with deterioration in vision, vascular abnormalities in the eye and the systemic severity of the disease. The results will indicate if the degree and progression of thickening are reliable early indicators that advanced edema requiring prompt treatment will develop within a short time. The second study will consist of following 100 patients undergoing laser treatment to assess the relation between retinal thickening and the ultimate visual outcome. If the two studies will indicate that the degree of retinal thickening, then quantitative and sensitive monitoring of the retinal thickness would hold promise to reduce the incidence of this common cause of vision loss. In parallel with the clinical research, the RTA will be further developed into an instrument well-suited for the clinic and easily operable by existing ophthalmic personnel.